Acquisition of maternal antibodies is critical to the immunologic defense of the newborn. In suckling rats and mice, a receptor for the Fc region of immunoglobulin G (IgG) transports IgG from milk across the intestinal epithelium into the blood (FcRn, n for neonate). FcRn is very similar in structure to class I major histocompatibility complex proteins. Late in gestation, the same receptor appears to transport IgG across the fetal yolk sac in these species. Prenatal transport accounts for only a small fraction of the maternal IgG that rodents receive, but in humans IgG transport occurs mostly, and perhaps only, before birth. A homolog of FcRn is expressed in human placenta and is likely to mediate materno-fetal IgG transport. A major goal of the proposal is to determine whether human FcRn is indeed responsible for placental IgG transport. Specific aims toward this goal include the localization of FcRn in human placenta by immunocytochemistry and in situ hybridization. The presence of the receptor in one or both of the cellular barriers between maternal and fetal blood, the syncytiotrophoblast and fetal vessel endothelia, would be consistent with a transport role. The specificity of human FcRn for different human IgG subclasses, measured by a competitive binding assay, will be compared with the relative efficiency of placental transport of these subclasses. A relatively low affinity for IgG2, which is transported poorly, would suggest that FcRn has a role in IgG transport from mother to fetus. The molecular mechanism of transcytosis of IgG by FcRn is not known. The second major goal of this proposal is to determine how this process occurs. Specific aims related to this goal include the characterization of the trafficking of rat FcRn functionally expressed in the polarized Madin-Darby canine kidney (MDCK) cell line. Sorting signals in the cytoplasmic region of rat FcRn will be located by site-directed mutagenesis, and expression of mutant receptors in MDCK cells. Proteins that interact with sorting signals will be sought using a yeast two-hybrid method. The funding of this RCDA would allow a 50% increase in the time available to the P.I. for research. This additional time would be particularly valuable now, because the emphasis of the project is shifting from the molecular biology methods in which the laboratory personnel have been trained to cell biology techniques that the P.I. must teach. In order to complete the proposed research it will also be necessary for the P.I. to learn new experimental methods. The reduction in non-research responsibilities would facilitate the timely completion of the project. Lastly, the proposed work almost completes the studies that follow directly from the discovery by the P.I. of MHC class I-related FcRs, and additional time will be essential to consider and plan future research directions.